In the oil and gas exploration and production industry; a wellbore or borehole of an oil or gas well is typically drilled from surface to a first depth and lined with a steel casing which is cemented in place. The borehole is then extended and a further section of smaller diameter casing is located in the extended section and also cemented in place. This process is repeated until the wellbore has been extended to a certain depth, and tubing known as a liner is then typically located in the borehole, extending from the deepest casing section (the casing ‘shoe’) to a producing formation. The well is then completed by locating a string of production tubing within the casing/liner and perforating the liner such that well fluids may flow from a producing formation, into the liner, and through the production tubing to surface.
The location of a liner extending from a casing shoe typically involves hanging the liner from the casing shoe using a liner hanger. Liner hangers include mechanical slips or the like which are selectively activated downhole to grip the internal wall of the casing, so that the liner may be suspended from the casing shoe and then cemented in place. Such liner hangers are typically run into the casing on a workstring carrying a liner setting tool, and are hydraulically actuated on exposure to fluid above a specified setting pressure, which urges the slips outwardly into engagement with the casing wall. These setting pressures are typically significantly higher than the hydrostatic pressure at depth within the well borehole, to prevent premature activation of the hanger.
To activate the hanger and thus to set the liner within the casing, it is necessary to close off fluid flow down through the workstring and the liner using the setting tool, so that the pressure of the fluid above the setting tool can be raised above the determined level necessary to activate the hanger. Currently, this is typically achieved by providing a setting tool including a ball seat which receives a ball dropped into the workstring from surface. The ball passes down into the setting tool, and lands on the ball seat to close fluid flow through the liner. The fluid above the tool is then pressured up, activating the hanger to set the liner. A further increase in fluid pressure acting on the ball (and thus on the ball seat) shears pins holding the ball seat in place. The ball seat is then carried down to a position where the seat is desupported, so that the ball can pass through the seat and exit the setting tool. Fluid flow through the liner has then been re-opened and the setting tool and workstring can be recovered to surface.
Whilst the above described tools and methods are effective at setting liner hangers within a casing, such ball seats are typically rated to shear at a relatively high pressure, typically 3000 psi or more, in order to prevent premature shearing out of the ball seat. At these high pressures, when the ball shears the seat out, there is an undesirable hydraulic shock imparted to the workstring carrying the liner/setting tool, and indeed to the surrounding rock formations, which can cause serious damage.
Similar such problems are encountered where other downhole tools are provided with ball seats that shear out at such high pressures, and indeed other types of ball seats and balls which operate at high pressures. Such alternative structures include those with deformable balls or ball seats, which permit blow-through of a ball at determined pressures. Typical such alternative tools include those utilised to selectively circulate fluid into an annulus between a workstring and the wall of a casing, to assist in a casing cleaning procedure, although it will be understood that many different types of downhole tool are activated in this way.
It is therefore amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.